Data transmission system and method using sound waves

ABSTRACT

There is described a method of transmitting data by means of acoustic waves between a transmitter device ( 2 ) and a receiver device ( 1 ) wherein the transmitter device has a first electroacoustic transducer ( 26 ) for transmitting an acoustic carrier wave at one or more frequencies and means for modulating the acoustic carrier wave as a function of data to be transmitted and the receiver has a second electroacoustic transducer ( 18 ) for receiving the acoustic carrier wave modulated by the transmitter device and means for demodulating the acoustic carrier wave and extracting the transmitted data therefrom. The first and second electroacoustic transducers each have a determined bandwidth and a determined frequency response characteristic. The frequency of the acoustic carrier wave is varied during a determined time period to sweep a determined range of frequencies situated in the band common to the first and second electroacoustic transducers so that the frequency of the transmitted acoustic carrier wave does not coincide at any time with a peak or a trough of the frequency response characteristic of the first or the second electroacoustic transducer. There is also described a data transmission system for implementing the above method.

The present invention relates generally to systems and methods fortransmitting data by means of acoustic waves.

In the context of the present description, the expression “acousticwave” means an elastic wave producing a sound that is audible orinaudible depending on its wavelength or, in other words, an elasticwave whose wavelength in the propagation medium concerned corresponds toan infrasound or sound frequency or even an ultrasound frequency.

Systems employing acoustic waves to transmit data are known in the art.For example, U.S. Pat. No. 4,242,745 describes a timepiece provided withan electroacoustic transducer for receiving data transmitted bymodulating an acoustic carrier wave generated by an external transmitterdevice.

U.S. Pat. No. 4,320,387 describes a portable device and a method forcommunicating data by means of acoustic waves. This document proposes inparticular to transmit data by ultrasound using an electroacoustictransducer. It proposes in particular to transmit data using a techniqueof frequency modulation of the acoustic carrier wave known as frequencyshift keying (FSK).

U.S. Pat. No. 5,719,825 and U.S. Pat. No. 5,848,027 both describe asystem for recording and processing personal data of a user (for examplethe physical activity of an athlete) including in particular a dataprocessing terminal and an electronic timepiece able to communicate bymeans of acoustic waves. More particularly, the timepiece is providedwith an electroacoustic transducer (piezoelectric element) fortransmitting personal data of the user to the data processing terminal,which is itself provided with a microphone for receiving acoustic wavesgenerated by the timepiece.

The document EP 1 075 098, in the name of the present Applicant,describes an acoustic signal converter circuit and a method ofbidirectional communication by means of acoustic waves for exchangingdata between two timepieces or between a data processing terminal and atimepiece.

Finally, the document WO 2001/10064, also in the name of the presentApplicant, describes a system for acoustic communication between aportable unit and a communication terminal.

The last two documents mentioned above propose in particular to use theexisting audio installation (loudspeakers and sound card) of a dataprocessing terminal to transmit the required data to a portable unit bymeans of acoustic waves. One advantage of this solution is that it isnot necessary to provide the data processing terminal with any kind ofdevice dedicated exclusively to transmitting and/or receiving data.

The typical solution envisaged until now for transmitting dataacoustically, in particular from a data processing terminal to aportable unit, consists in generating an acoustic carrier wave at adetermined frequency and modulating that acoustic carrier wave as afunction of the data to be transmitted. The modulation of the acousticcarrier wave as a function of the data can, for example, consist inamplitude modulation, frequency modulation or phase modulation of theacoustic carrier wave using modulation techniques known in the art.

However, it is noticeable that the loudspeakers with which off the shelfdata processing terminals are typically equipped are low cost devicesand therefore have highly irregular frequency response characteristics.Measurements carried out on a sample of loudspeakers available off theshelf have shown strong variations in the amplitude of the signal as afunction of frequency (often of greater than ±10 dB). In fact, mostloudspeaker systems usually proposed for use with personal computers arenot intended to reproduce high fidelity sound and their response curveis therefore very irregular. This irregular response curve isessentially due to the fact that the acoustic impedance of the speakervaries rapidly with frequency and has very marked extrema at its naturalfrequencies, leading to peaks and troughs in the response curve of thesystem. It is also noticeable that the amplitude distortion problem isaggravated if the distance between the loudspeaker and the portable unitis short.

A drawback associated with using existing loudspeaker systems istherefore that it is not possible to assure highly reliable transmissionof data by means of acoustic waves as the frequency of the acousticcarrier wave may coincide with a peak or a trough in the frequencyresponse of the loudspeaker, regardless of the modulation technique usedto transmit the data.

A solution must therefore be found which allows increasing thereliability of the transmission of such a data transmission system usingacoustic waves. The object of the present invention is to propose onesuch solution.

To this end, the present invention proposes a method with the featuresset out in claim 1 for transmitting data by means of acoustic wavesbetween a sender transmitter device and a receiver device.

The present invention also consists in a system with the features setout in claim 9 for transmitting data by means of acoustic waves forimplementing the above transmission method.

Advantageous embodiments of the present invention form the subjectmatter of dependent claims.

Thus, according to the invention, the frequency of the acoustic carrierwave is varied, during a determined time period, to sweep a determinedrange of frequencies in the band common to the first and secondelectroacoustic transducers respectively equipping the transmitter andreceiver devices. This assures that the frequency of the transmittedacoustic carrier wave does not at any time coincide with a peak or atrough of the frequency response characteristic of the first or secondelectroacoustic transducer.

Data is transmitted by appropriate modulation (in particular amplitudemodulation) of the acoustic carrier wave, complemented by frequencymodulation of the acoustic carrier wave with the essential object ofwidening the send frequency spectrum of the acoustic signal in thebandwidth of the transmitter and/or receiver device. Varying thefrequency of the acoustic carrier wave in this manner ensures that thefrequency of the transmitted signal does not at any time coincide with apeak or a trough in the frequency response of the acoustic system whichis employed. It is therefore clear that, according to the invention, twomodulations of the acoustic carrier wave are superposed, one to transmitdata and the other, in this instance involving variation or modulationof the frequency of the acoustic carrier wave, to ensure sufficientspectral diversity of the acoustic carrier wave.

In one particularly advantageous embodiment of the present inventionthat has proved particularly effective, the frequency of the acousticcarrier wave generated by the acoustic transducer of the transmitterdevice is varied by a frequency modulation technique using one or moremodulating signals. It has been found that using this solution leads tovery high reliability of data transmission.

In one embodiment of the data transmission system, there is alsoprovision for storing the acoustic carrier wave in the form of asuccession of samples stored in a table. This greatly simplifies thegeneration of the modulated acoustic carrier wave in that it issufficient to consult the table and to generate the acoustic carrierwave on the basis of the succession of stored samples. It is thereforenot necessary to provide the transmitting system with dedicatedelectronic means. In fact it is sufficient to provide a simple dataprocessing application known as a “plug-in” to implement the inventionon a data processing terminal.

Generally speaking, it is clear that two methods may be used to generatethe acoustic carrier wave: either direct generation according to whichthe acoustic carrier wave is generated by means of a mathematicalfunction implemented when sending the data, or indirect generationaccording to which a predefined “wave table” is stored and may be readat the time of resituating the acoustic carrier wave.

It will be noted that a further advantage of the present invention isthat implementing the invention necessitates no modification of thereceiver unit, which operates in a manner that is in every respectanalogous to what was the situation previously. Implementation of theinvention is therefore particularly simple and inexpensive.

Other features and advantages of the present invention will become moreclearly apparent on reading the following detailed description ofembodiments of the invention, which is given by way of non-limitingexample only and illustrated by the appended drawings, in which:

FIG. 1 is a diagram of a system for communicating data by means ofacoustic waves between a data processing terminal and a portable unitsuch as a watch;

FIG. 2 is a timing diagram of an acoustic carrier wave generatedaccording to a first implementation mode of the invention in which thefrequency of the acoustic carrier wave is varied in substantially linearfashion over a determined range of frequencies;

FIG. 3 is a timing diagram of an acoustic carrier wave generatedaccording to another implementation mode of the invention in which thefrequency of the acoustic carrier wave is varied by means of frequencymodulation technique employing two modulating signals;

FIG. 4 is a diagram of the frequency spectrum resulting from continuoussending of the acoustic carrier wave according to the implementationmode of FIG. 3;

FIG. 5 is a diagram of the transmission of a determined sequence of bitsby amplitude modulating of the acoustic carrier wave frequency-modulatedaccording to the implementation mode of FIG. 3; and

FIG. 6 is a diagram of a succession of samples defining the FIG. 3acoustic carrier wave over the time taken to send one bit.

FIG. 1 is a diagram of a system for communicating data by means ofacoustic waves between a data processing terminal and a portable unitdesignated as a whole by the numeral references 1 and 2 respectively.The portable unit 1 may advantageously take the form of a wristwatchthat may be worn on the wrist of a user, for example.

The data processing terminal 2 may be an off the shelf personal computer(PC) comprising means for emitting acoustic signals conveyinginformation. In the schematic example depicted in FIG. 1, these meanstypically take the form of a sound card 24 disposed inside the personalcomputer, one or more loudspeakers 26 and a microphone 28.

It will be remembered that one advantage of the system shown in FIG. 1is that it is not necessary to modify the structure of the dataprocessing terminal or to add to it transmitting components specific tothe type of wireless link used. To implement the invention, it issufficient to load into the computer a program enabling it to modulatethe acoustic signal so that this signal may afterwards be decodedcorrectly by the portable unit 1.

If the data processing terminal 2 sends an acoustic signal conveyinginformation by means of its loudspeaker(s) 26, the signal is immediatelypicked up by the receiver means of the portable unit 1. These receivermeans take the form of a bidirectional electroacoustic transducer 18which serves at the same time as a microphone (acoustic receiver) and aloudspeaker (acoustic transmitter). In receive mode, thiselectroacoustic transducer 18 transforms the incoming acoustic signalinto an electrical signal which is then converted by converter means ofthe portable unit 1 into data to be processed by processing means ofthis unit in order to extract therefrom useful information carried bythe acoustic signal. In the FIG. 1 example, the conversion means of theportable unit 1 comprise an amplifier 10 for amplifying the electricalsignal produced by the electroacoustic transducer 18 and a demodulationcircuit (demodulator) 12 connected to the signal amplifier and adaptedto demodulate the received signal and to pass the demodulated signal toan input of a microcontroller 14 constituting the processing means ofthe portable unit. The information carried by the acoustic signal sentby the data processing terminal 2, demodulated by the demodulator 12 andprocessed by the microcontroller 14, may be stored in a memory 16 of theportable unit 1 and/or displayed on a display device 15, for example aliquid crystal display. A battery 11, which may be a rechargeablebattery, supplies the portable unit 1 with electrical power.

In the context of sending data by acoustic way using the portable unit,this latter unit is further equipped with conversion and sending meansfor converting data supplied by the processing means of the portableunit into a modulated acoustic signal and sending that signal. As shownin FIG. 1, these conversion means comprise a modulation circuit(modulator) 13 which drives the transmitter means, namely theelectroacoustic transducer 18, via a driver circuit 17. The processingmeans of the portable unit 1, i.e. the microcontroller 14, control themodulation circuit 13 as a function of the data to be transmitted, whichis typically stored in the memory 16 of the portable unit 1.

It will further be noted that the microcontroller 14 in FIG. 1 typicallycomprises encoding and decoding means (respectively upstream anddownstream of the modulator 13 and the demodulator 12). Also, themodulator 13 and/or the demodulator 12 may in practice constitute anintegral part of the functions of the microcontroller 14.

The detailed structure of the electroacoustic transducer and theassociated processing and conversion means are not described in detailhere. See, for example, the documents EP 1 075 098 and WO 2001/10064cited in the preamble and incorporated herein by reference. Thosedocuments propose in particular modifying a sound generator circuit usedconventionally to generate alarms into a bidirectional converter circuitable to convert a modulated acoustic signal into an electrical signaland vice-versa.

It should be noted that the communication system shown in FIG. 1 isadapted to provide bidirectional communication between the dataprocessing terminal and the portable unit, the loudspeaker(s) 26 beingused to transmit data from the personal computer 2 to the portable unit1 and the microphone 28 being used to receive data transmitted by theportable unit 1. The remainder of the description is more particularlyconcerned with transferring data from the data processing terminal 2 tothe portable unit 1.

As mentioned in the preamble, one drawback of the prior art solutions isthat the frequency of the acoustic carrier wave used to transmit datamay coincide with a trough or a peak of the frequency response of theloudspeaker. This problem arises regardless of the type of modulationused to code the information. In the case of amplitude modulation, theinformation is coded by varying the amplitude of the acoustic carrierwave, which is transmitted at a determined frequency that may thuscoincide with an irregularity in the frequency response of theloudspeaker. The same applies to phase modulation, where information iscoded by varying the phase of the signal. Finally, in the case offrequency modulation, in which information is coded by varying thefrequency of the acoustic carrier wave, the frequency of the modulatedacoustic carrier wave can at least partly coincide with an irregularityin the frequency response of the loudspeaker, and a portion of the datamay consequently be lost.

According to the invention, the choice is nevertheless made to introducehigh spectral diversity into the acoustic carrier wave by varying thefrequency of the carrier wave in a range of determined frequencies inthe bandwidth common to the electroacoustic transducer of theloudspeaker and the electroacoustic transducer of the portable unit. Thedata to be transmitted are transmitted by appropriate modulation of theacoustic carrier wave that is itself frequency-modulated. The choice ofthe modulation used to transmit the data is dictated by the conditionthat there must be no or little interference between the two modulations(the modulation used to transmit the data and the frequency modulationadopted to ensure sufficient spectral diversity of the acoustic carrierwave).

The simplest solution is to use amplitude modulation of the acousticcarrier wave to transmit the data in addition to frequency modulation ofthis acoustic carrier wave. In this case, it will be noted that it isnevertheless necessary to choose frequency modulation parametersensuring, firstly, as already mentioned, sufficient spectral diversityof the acoustic carrier waves and, secondly, that the envelope of theacoustic signal is affected as little as possible.

An alternative to amplitude modulation might be frequency modulation. Itwill be noted that in this case decoding the information becomes morecomplex because the frequency modulation used to transmit the data issuperposed on the frequency modulation used to spread the frequencyspectrum in the useful bandwidth. In this case, an I/Q demodulator (withsignals in phase quadrature) could allow to discriminate the phase orthe frequency of the carrier wave.

In the remainder of the description, it will be assumed for the sake ofsimplicity that the data is transmitted by amplitude modulation of theacoustic carrier wave. To be more specific, the basic principle is thatthe acoustic carrier wave has a determined non-zero amplitude level overthe bit sending time when the bit value is equivalent to a first logiclevel (for example “1”) and a zero amplitude level over the bit sendingtime when the bit value is equivalent to the second logic level (forexample “0”). For example, one can refer to FIG. 5 which shows a diagramof sending a sequence of bits using the abovementioned technique.

Note that here there is a specific amplitude modulation mode and thatother amplitude modulation modes may perfectly well be envisaged, forexample a modulation mode in which a bit at “1” is transmitted in theform of a succession of two half-periods in which the amplitude of theacoustic carrier wave is first non-zero and then zero and conversely inwhich a bit at “0” is transmitted in the form of a succession of twohalf-periods in which the amplitude of the acoustic carrier wave isfirst zero and then non-zero (this is commonly referred to as Manchestermodulation or coding).

The solution for achieving great spectral diversity of the acousticsignal in a determined range of frequencies consists in varying thefrequency of the acoustic carrier wave in the useful bandwidth, i.e. thebandwidth common to the electroacoustic transducer of the loudspeakerand the electroacoustic transducer of the portable unit. By way ofpurely illustrative and non-limiting example, it has been determinedthat the useful bandwidth of the system could correspond to a range offrequencies from approximately 2700 Hz to approximately 4000-4500 Hz,(i.e. a bandwidth of the order of 1.5 kHz), this bandwidth beingessentially determined by the characteristics of the electroacoustictransducer employed in the portable unit and by the construction of theportable unit.

A first solution that may be envisaged consists in varying the frequencyin the useful band in a substantially linear manner. In this case, theacoustic carrier wave may be expressed in the following analytical form:CARRIER(t)=sin(2π(f 0+Δf·(t/Tbit))·t+alpha)  (1)in which f0 is the starting frequency of the frequency sweep, Δfcorresponds to half of the frequency band to be swept, Tbit is the bitsending time, and alpha is an appropriate phase-shift for ensuring thecontinuity of the acoustic carrier wave from one bit to another (thisphase-shift may be neglected if appropriate). This phase-shift alpha maybe expressed in the following manner:alpha=(2π·(f 0+Δf)·Tbit)·(N−1)  (2)in which N corresponds to the N^(th) bit concerned.

FIG. 2 represents the acoustic carrier wave conforming to the aboveequation (1). In this figure, an arbitrary bit sending time ofapproximately 7.8 ms (to be exact: 1/128=7.8125 ms) and values for theparameters f0 and Δf of 3000 Hz and 1000 Hz, respectively, have beenchosen. Note that in this figure the phase alpha of the signal is alsoadjusted from one bit to the next.

Spectral analysis of the acoustic carrier wave generated in accordancewith the above principle shows that the range of frequencies over whichthe acoustic carrier wave is generated essentially extends from theselected frequency f0 over a bandwidth equivalent to 2×Δf. In the abovenumerical example, where the values of f0 and Δf are respectively 3000Hz and 1000 Hz, the spectrum of the generated acoustic carrier wave liesessentially in a band of frequencies from 3000 Hz to 5000 Hz.

Δn alternative solution to the solution consisting in varying thefrequency of the acoustic carrier wave in a linear manner over adetermined frequency range consists in varying the frequency of theacoustic carrier wave by frequency modulation technique using one ormore modulating signals. In the case of frequency modulation using twomodulating signals, the acoustic carrier wave may be expressed in thefollowing analytical form:CARRIER(t)=sin(2π·f 0·t+Δ1/f 1 sin(2π·f 1·t)+Δ2 /f 2 sin(2π·f 2·t))  (3)in which f0 is the centre frequency of the acoustic carrier wave, f1 andΔ1 are respectively the frequency and the maximum deviation of the firstmodulating signal and f2 and Δ2 are respectively the frequency and themaximum deviation of the second modulating signal. As previouslymentioned with reference to equation (1), although this parameter is notnecessary, it may be further envisaged that the definition of theacoustic carrier wave referred to above includes a phase shift selectedto ensure continuity of the acoustic carrier wave from one bit toanother.

FIG. 3 shows the acoustic carrier wave conforming to the above equation(3). In this figure, the bit sending time Tbit is again equivalent toapproximately 7.8 ms. Respective values of the parameters f0, f1, Δ1, f2and Δ2 in this example are 3331 Hz, 1000 Hz, 200 Hz, 600 Hz and 120 Hz.Note that the choice of the parameters f0, f1, Δ1, f2 and Δ2 is dictatedby certain constraints. Thus the centre frequency f0 is defined as afunction of the useful bandwidth of the system and is substantially inthe middle of that useful bandwidth. The modulation parameters f1, Δ1,f2 and Δ2 are chosen as a function of the bit sending time Tbit and theuseful bandwidth of the system, the essential constraint being to ensuresufficient spectral diversity of the acoustic carrier wave in the usefulbandwidth.

The parameters f0, f1, f2, Δ1 and Δ2 selected vary the bandwidth of thefrequency spectrum of the acoustic carrier wave and the number and thepositions of the frequency peaks of the acoustic carrier wave. FIG. 4shows by way of illustrative example the spectrum resulting fromcontinuous repetition of the FIG. 3 acoustic carrier wave when therepetition period is 7.8 ms. Note in particular a frequency peak at thecentre frequency of 3331 Hz and additional peaks at 2331 Hz, 2731 Hz,3931 Hz and 4331 Hz, as well as other frequency peaks of lowerintensity.

From a qualitative point of view, it is found that the second solutionreferred to above, in which the acoustic carrier wave is modulated byone or more modulating signals, gives better results. Note that, as thedata is transmitted by amplitude modulation of the acoustic carrierwave, the modulation of the frequency of the acoustic carrier wave whichis adopted must be such that the envelope of this acoustic carrier waveremains substantially constant (i.e. remains substantially unaffected)for a given amplitude modulation level, so as not to interfere much orat all with the transmission of data.

In the context of implementing the invention on a data processingterminal equipped with one or more loudspeakers, it will be advantageous(in particular in relation to limiting the computation load of the dataprocessing terminal), to store the acoustic carrier wave in the form ofa succession of predetermined samples. In particular, a succession ofsamples representative of the acoustic carrier wave over the duration ofa bit must be memorised, for example in the form of a table stored inthe memory of the data processing terminal. To generate the acousticwave, it is then sufficient to consult the stored table to generate theportion of the acoustic carrier wave corresponding to the bit sendingtime and to repeat that operation for each bit to be transmitted. Thisacoustic carrier wave is then modulated as a function of the data to betransmitted. In the particular situation where the data is transmittedby amplitude modulation in accordance with the principle referred toabove in relation to FIG. 5, it is clear that, properly speaking, theacoustic carrier wave is generated only when it is necessary to transmita bit at “1”, the acoustic carrier wave having a zero amplitude when abit at “0” is transmitted.

It will be appreciated that a sampling frequency of 44.1 kHz istypically adopted for sampling audio signals on personal computers. Fora bit sending time Tbit of approximately 7.8 ms, for example, theacoustic carrier wave can therefore be represented by a set of 344successive samples. FIG. 6 represents the FIG. 3 acoustic carrier wavesampled at 44.1 kHz over one bit sending time.

It is generally clear that various modifications and/or improvementsthat will be evident to the person skilled in the art may be made to theembodiments described herein without departing from the scope of theinvention as defined by the appended claims. In particular, the presentinvention is not limited to the two implementing modes described above,in which the frequency of the acoustic carrier wave is varied in asubstantially linear manner or by a frequency modulation techniqueemploying a plurality of modulating signals. Any other appropriate formof modulation may be adopted to vary the frequency of the acousticcarrier wave provided that this modulation ensures sufficient spectraldiversity of the acoustic carrier wave in the required bandwidth.

Finally, the present invention is not limited to the implementation ofthe proposed method in a system including at least a data processingterminal and a portable unit. The proposed transmission method appliesto any system for transmitting data by means of acoustic waves in whichthe electroacoustic transducer of the transmitter device has anirregular frequency response. Similarly, the same principle may beadopted to prevent the frequency of the acoustic carrier wave coincidingwith an irregularity in the frequency response of the electroacoustictransducer used in the receiver (for example the microphone of the dataprocessing terminal). The proposed transmission method could thereforebe implemented in the portable unit to improve the reliability of datatransmission from the portable unit to the data processing terminal.

Moreover, the configuration of the portable unit shown in FIG. 1 employsa bidirectional electroacoustic transducer. It is clear that twoelectroacoustic transducers respectively dedicated to sending andreceiving data could be used. Finally, the present invention alsoapplies to a unidirectional data transmission system.

1-10. (canceled)
 11. A method of transmitting data by means of acousticwaves between a transmitter device and a receiver device, saidtransmitter device having a first electroacoustic transducer fortransmitting an acoustic carrier wave at at least one frequency andmeans for modulating said acoustic carrier wave as a function of data tobe transmitted, said receiver device having a second electroacoustictransducer for receiving said acoustic carrier wave modulated by thetransmitter device and means for demodulating said acoustic carrier waveand extracting the transmitted data therefrom, and said first and secondelectroacoustic transducers each having a determined bandwidth and adetermined frequency response characteristic, wherein the frequency ofsaid acoustic carrier wave is varied during a determined time period tosweep a determined range of frequencies situated in the bandwidth commonto said first and second electroacoustic transducers so that thefrequency of the transmitted acoustic carrier wave does not coincide atany time with a peak or a trough of the frequency responsecharacteristic of said first or said second electroacoustic transducer.12. The method according to claim 11, wherein said modulation means ofthe transmitter device are amplitude modulation means and in that thefrequency of said acoustic carrier wave is varied so that the envelopeof this acoustic carrier wave remains substantially constant for a givenmodulation amplitude level.
 13. The method according to claim 11,wherein the frequency of said acoustic carrier wave is varied in asubstantially linear manner over said determined range of frequencies.14. The method according to claim 12, wherein the frequency of saidacoustic carrier wave is varied in a substantially linear manner oversaid determined range of frequencies.
 15. The method according to claim11, wherein the frequency of said acoustic carrier wave is varied bymeans of a frequency modulation technique employing one or moremodulating signals.
 16. The method according to claim 12, wherein thefrequency of said acoustic carrier wave is varied by means of afrequency modulation technique employing one or more modulating signals.17. The method according to claim 15, wherein the frequency of saidacoustic carrier wave is varied by means of a frequency modulationtechnique employing two modulating signals and has the shape of the typedefined by the following equation:CARRIER(t)=sin(2π·f ₀ ·t+Δ1/f 1 sin(2π·f 1 ·t)+Δ2 /f 2 sin(2π·f 2·t))  (3) in which CARRIER(t) is the expression for said acousticcarrier wave as a function of time, f0 is the centre frequency of saidacoustic carrier wave, f1 and Δ1 are respectively the frequency and themaximum deviation of the first modulating signal and f2 and Δ2 arerespectively the frequency and the maximum deviation of the secondmodulating signal.
 18. The method according to claim 16, wherein thefrequency of said acoustic carrier wave is varied by means of afrequency modulation technique employing two modulating signals and hasthe shape of the type defined by the following equation:CARRIER(t)=sin(2πf₀ ·t+Δ1/f 1 sin(2π·f 1·t)+Δ2/f 2 sin(2π·f 2·t))  (3)in which CARRIER(t) is the expression for said acoustic carrier wave asa function of time, f0 is the centre frequency of said acoustic carrierwave, f1 and Δ1 are respectively the frequency and the maximum deviationof the first modulating signal and f2 and Δ2 are respectively thefrequency and the maximum deviation of the second modulating signal. 19.The method according to claim 15, wherein said acoustic carrier wave isan acoustic wave that has a centre frequency of the order of 3000 Hz to3500 Hz which is frequency modulated by said modulating signals.
 20. Themethod according to claim 16, wherein said acoustic carrier wave is anacoustic wave that has a centre frequency of the order of 3000 Hz to3500 Hz which is frequency modulated by said modulating signals.
 21. Themethod according to claim 17, wherein said acoustic carrier wave is anacoustic wave that has a centre frequency of the order of 3000 Hz to3500 Hz which is frequency modulated by said modulating signals.
 22. Themethod according to claim 18, wherein said acoustic carrier wave is anacoustic wave that has a centre frequency of the order of 3000 Hz to3500 Hz which is frequency modulated by said modulating signals.
 23. Themethod according to claim 15, wherein the data to be transmittedcomprises a succession of bits transmitted by amplitude modulation ofsaid acoustic carrier wave between first and second determined amplitudelevels and wherein the parameters of frequency modulation of saidacoustic carrier wave are selected so that the frequency spectrum of theacoustic carrier wave resulting from said frequency modulation coverssubstantially all of the band common to the transmitter and receiverdevices.
 24. The method according to claim 16, wherein the data to betransmitted comprises a succession of bits transmitted by amplitudemodulation of said acoustic carrier wave between first and seconddetermined amplitude levels and wherein the parameters of frequencymodulation of said acoustic carrier wave are selected so that thefrequency spectrum of the acoustic carrier wave resulting from saidfrequency modulation covers substantially all of the band common to thetransmitter and receiver devices.
 25. The method according to claim 11,wherein said acoustic carrier wave is stored in said transmitter devicein the form of a succession of samples stored in a table.
 26. A systemfor transmitting data by means of acoustic waves for implementing thetransmission method according to claim 11, wherein this systemcomprises: a data processing terminal associated with at least oneacoustic transmitter device having a first electroacoustic transducerfor transmitting said acoustic carrier wave, and at least a portabledevice provided with an acoustic receiver device having a secondelectroacoustic transducer for receiving said acoustic carrier wave. 27.The system for transmitting data according to claim 26, wherein saidacoustic carrier wave is stored in said data processing terminal in theform of a succession of samples stored in a table and wherein said dataprocessing terminal comprises software means for: consulting said table,generating an acoustic carrier wave based on said succession of samples,and modulating the acoustic carrier wave as a function of the data to betransmitted.